I've been taking Lithium on and off for almost 20 years (consistent usage for the last 15 years). Your body takes time to adapt. I found that the extended release (Lithium Carbonate ER) helped immensely. No bouncing up and down on my levels.

Agree with Moreta. Get your labs drawn regularly. At 600mg, you're at the very low spectrum. Mean Lithium dose ranges between 900mg and 1800mg. I've heard of higher doses than 1800mg, but I don't think that is good as it is near toxic levels. Adding on an anti-seizure or atypical medication is the best option in my book.